The present invention relates to a merged bipolar and metal oxide transistor integrated circuit device including a method of fabrication thereof. The method is applicable to incorporating complementary metal oxide transistors together with a bipolar transistor on a semiconductor substrate in a single merged flow.
Semiconductor manufacturers have wanted to combine bipolar and complementary metal oxide semiconductor (CMOS) transistors in integrated circuits in order to obtain the rapid switching ability and high current drive of the bipolar transistor together with the comparatively low energy consumption of the CMOS transistors. However, the general approach has been to manufacture the merged devices serially in order to avoid compromising diffusion profiles which result in poorer performance. But serial processing is lengthy and proliferates the number of masking steps which in turn reduces yield. Moreover, serial processing is susceptible to the problem of later process steps altering the results of earlier process steps.
The alternative to serial processing is merged or parallel processing in which some or all of the steps required to build one device are used simultaneously to build others thereby reducing the number of masking levels and processing steps. Unfortunately, the process recipes used to date do not always produce optimum performance in all structures simultaneously. In some cases a process recipe that optimizes the performance of some of the devices actually prevents other devices from functioning in any useful manner at all.
A process for merging bipolar and MOS transistors is disclosed in U.S. Pat. No. 4,536,945 issued to Gray et al on Aug. 27, 1985 and assigned to National Semiconductor Corporation. In this case a bipolar transistor and a p-channel transistor are formed in an n-type epitaxial layer while an N-channel transistor is formed on a retrograde p-well. Each transistor is isolated by a thick oxide region extending down to the substrate. The Gray device discloses no means for inhibiting "latch up" of its p-channel transistor caused by leakage current flowing to the substrate.
Gray forms a retrograde p-well before formation of the oxide isolation regions used to separate the transistors. This is done by diffusing together a second impurity deposited on the substrate and a third impurity deposited on the epitaxial layer. The latter step is followed by other anneal steps in addition to that for forming the oxide isolation regions. These anneal steps cause upward diffusion of the diffused N-type region at the bottom of the bipolar transistor making the thickness of the epitaxial region uncertain and difficult to control. The latter results make it difficult, in turn, to control the transistor breakdown voltage.
A second problem caused by the foregoing anneal steps is the depletion of boron at the surface caused by leaching or depletion of the boron by silicon oxide. This depletion makes the threshold voltage difficult to control.
The use of an indirect moat by Gray et al. and reliance on the oxide isolation regions to establish moat boundaries results in a lower limit to the spacing between transistors which is achievable (typically of the order of 10 microns). Secondly, due to the charge stored on the oxide-silicon interface a relatively significant leakage normally results along this interface.
Third, use of a long high temperature collector diffusion in Gray creates problems in controlling transistor gain and breakdown voltage. Such prolonged high temperature applications cause all of the diffused areas to change and further deplete the surface concentration of Boron and causes upward diffusion of the N-type layer at the substrate surface. In addition, there is no provision for avoiding the formation of polysilicon filaments folowing etching due to the steep topography created by the `birdsheads` at the top of the oxide isolation regions. These filaments can cause shorting problems.
It is therefore an object of the invention to provide an improved method of making merged bipolar and MOS devices. In particular it is an object of this invention to provide a method of making an improved merged bipolar and CMOS device.